In recent years, in a mobile communication system or a satellite communication system, there is considered a code division multiplex connection communication scheme using a spread spectrum scheme serving as one of the traffic transmission schemes of images, voice, data, or the like.
The direct sequence scheme, which is one of the spread spectrum communication schemes, is a scheme in which a spread code sequence in a band considerably wider than that of an information signal is directly multiplied by the information signal to spread the information signal during communication.
An initial acquisition circuit is used to establish a code synchronization between a spread code sequence multiplied on transmission side and a spread code sequence used in a correlator on a reception side. In the following explanation, a code synchronization state is a state in which the phase of a spread code multiplied by a reception signal and the phase of a spread code used in a correlator are equal to each other in data demodulation.
FIG. 8 is a block diagram of a transmission device and FIG. 9 is a block diagram of a reception device of a spread spectrum communication system (to be referred to as the identical spread code multiplex SS system hereinafter), described in Japanese Patent Application Laid-Open No. 6-197096, in which spectrum spreading is performed on conventional parallel data by using identical spread codes, and, thereafter, multiplexing is performed with a time offset.
FIG. 8 is a block diagram showing a transmission device of a conventional identical spread code multiplex SS system. In FIG. 8, reference numeral 211 denotes a data generator; reference numeral 212 denotes a serial/parallel converter; reference numeral 213 denotes a clock generator; reference numeral 214 denotes a spread code generator; reference numerals 22(1) to 22(n) denote spread modulators; reference numerals 23(1) to 23(n) denote delays; reference numeral 241 denotes a multiplexer; reference numeral 242 denotes a frequency converter; reference numeral 243 denotes a power amplifier; and reference numeral 244 denotes a transmission antenna.
Operation of the transmission device of the conventional identical spread code multiplex SS system will be explained below. The data generator 211 of the transmission device of the conventional identical spread code multiplex SS system generates a digital information signal having a value of "1" or "-1". In the following explanation, a generation rate of digital information signals is called a bit rate, and the value of the bit rate of the digital information signals is expressed as R.sub.b. The digital information signal is converted by the serial/parallel converter 212 into parallel information signals of n channels. The multiplex number n is equal or smaller than a spread code length L [bit]. In addition, in the following explanation, a generation rate of parallel information signals in each channel is called a parallel bit rate, and the value of the parallel bit rate is expressed as R.sub.p (=R.sub.b /n). Thus, a spread code sequence used in the transmission device of the conventional identical spread code multiplex SS system is generated by the spread code generator 214, which generates values of "1" or "-1". The spread code sequence has a code length L [bit], and a clock frequency band of R.sub.p.times.L generated by the clock generator 213.
Codes such as M-sequence codes or Gold codes which are formed by a simple code forming circuit configuration in which autocorrelation and cross correlation between the codes are small, are used as the spread code sequence. In the following explanation, a clock rate generated by the clock generator 213 is called a chip rate R.sub.c (=R.sub.p.times.L), and a clock cycle having the chip rate R.sub.c is called a chip cycle T.sub.c (=1/R.sub.c). In the spread modulators 22(1) to 22(n), the parallel information signals of the n channels are multiplied by the spread code generated by the spread code generator 214 to generate parallel spread spectrum signals of n channels. Each parallel spread spectrum signal has the chip rate R.sub.c.
N different delay times {.tau..sub.1 T.sub.c, .tau..sub.2 T.sub.c, .tau..sub.3 T.sub.c, . . . , .tau..sub.n T.sub.c } are given in the delay units 23(1) to 23(n) to the parallel spread spectrum signal sequence of the n channels. In the following description, {.tau..sub.1, .tau..sub.2, .tau..sub.3, . . . , .tau..sub.n } are called delay coefficients, and the delay coefficients {.tau..sub.1, .tau..sub.2, .tau..sub.3, . . . , .tau..sub.n } are integers which satisfy 0.ltoreq..tau..sub.1 &lt;.tau..sub.2 &lt;.tau..sub.3 &lt; . . . &lt;.tau..sub.n &lt;L. Thereafter, all the delayed parallel spread spectrum signals of the n channels are added to each other by the multiplexer 241 to generate a multiplexed spread spectrum signal. Although the parallel information signals of the parallel spread spectrum signal sequence of the n channels are spectrum-spread by identical spread code sequences, since the parallel spread spectrum signal sequence is multiplexed with different delay times, crosscorrelation between the signals of data sequence in code synchronization of the data sequence have small values.
Further, the frequency of the multiplexed spread spectrum signal is converted into a radio frequency (RF) by the frequency converter 242. Thereafter, the power of the multiplexed spread spectrum signal is amplified by the power amplifier 243 to generate a multiplexed RF signal. The multiplexed RF signal is transmitted to a communication destination using the transmission antenna 244.
FIG. 9 is a block diagram showing a reception device of a conventional identical spread code multiplex SS system. In FIG. 9, reference numeral 311 denotes a reception antenna; reference numeral 312 denotes an RF amplifier; reference numeral 111 denotes a quasi-coherent detector; reference numeral 112 denotes a correlation value calculator; reference numeral 141 denotes an initial acquisition unit; reference numeral 313 denotes a parallel/serial converter; reference numerals 32(1) to 32(n) denote delay correcting units; and reference numerals 33(1) to 33(n) denote data demodulators. The quasi-coherent detector 111 comprises a voltage controlled oscillator (VCO) 341, a .pi./2-phase shifter 342, multiplexers 343 and 344, lowpass filters 345 and 346, and A/D converters 347 and 348, and the correlation calculator 112 comprises an in-phase correlation calculator 351 and an orthogonal correlation value calculator 352.
Operation of a reception device of a conventional identical spread code multiplex SS system will be described below. In FIG. 9, the RF amplifier 312 in the reception device of the conventional identical spread code multiplex SS system RF-amplifies a multiplexed RF signal transmitted from the transmission device of the conventional identical spread code multiplex SS system explained above received through the reception antenna 311. In the quasi-coherent detector 111, a local carrier wave having a frequency band of a chip rate R.sub.c output from the VCO 341 and the RF-amplified multiplexed RF signal are multiplied together by the multiplexer 343. A harmonic component is removed from the resultant signal by the lowpass filter 345. This signal is then converted into digital data by the A/D converter 347, so that an in-phase component of a complex spread spectrum signal included in the frequency band of the chip rate R.sub.c is generated.
Similarly, in the quasi-coherent detector 111, a local carrier wave having a chip rate R.sub.c .pi./2 phase shifted by the .pi./2-phase shifter 342 and the RF-amplified multiplexed RF signal are multiplied together by the multiplexer 344. An orthogonal component of the complex spread spectrum signal having the frequency band of the chip rate R.sub.c is generated through the lowpass filter 346 and the A/D converter 348. In the correlation calculator 112, the in-phase component and the orthogonal component of the complex spread spectrum signal are correlatively operated with identical spread code sequences that are multiplied by the multiplexed RF signal in the in-phase correlation calculator 351 and the orthogonal correlation value calculator 352 respectively, to calculate an in-phase correlation value and an orthogonal correlation value. Matched filters or the like are used in the in-phase correlation calculator 351 and the orthogonal correlation calculator 352. A symbol clock synchronized with the cycle of the spread code sequence multiplied by the multiplexed RF signal is generated in the initial acquisition unit 141 on the basis of the orthogonal correlation value and the in-phase orthogonal value.
Delay correction times {T.sub.p -.tau..sub.1 T.sub.c, T.sub.p -.tau..sub.2 T.sub.c, T.sub.p -.tau..sub.3 T.sub.c, . . . , T.sub.p -.tau..sub.n T.sub.c } (where T.sub.p is a parallel bit cycle and satisfies T.sub.p =1/R.sub.p) are given to the orthogonal correlation value and the in-phase correlation value in the delay correcting units 32(1) to 32(n) so that timing synchronization between the phase of the spread code sequence multiplied in the respective channels and the symbol clock can be established. Further, parallel demodulation data for every channel is calculated in the data demodulators 33(1) to 33(n) on the basis of the in-phase correlation value and the orthogonal correlation value calculated at a timing synchronized with the symbol clock. The multiplexed RF signal is generated by multiplexing the parallel information signal sequences multiplied by the identical spread codes.
However, since different delay times are given to the multiplexed RF signals multiplexed in the respective channels, crosscorrelation between the channels in data modulation have small values. For this reason, a demodulation operation can be performed on each channel. The parallel demodulation data is a signal having a parallel bit rate R.sub.p and having a value of "1" or "-1". Further, in the parallel/serial converter 313, demodulation data having a bit rate R.sub.b (=nR.sub.p) is generated from the parallel demodulation data having parallel bit rates R.sub.p of n channels. Thus, demodulation data can be extracted from the multiplexed RF signal by the procedure explained above.
The structure and operation of the initial acquisition unit 141 will be explained below.
FIG. 10 is a block diagram showing the initial acquisition circuit of a conventional direct sequence scheme as described in "Spread Spectrum Communication System" (by Yokoyama Mitsuo, Kagakugijyutsu Shuppan-sha) pp. 325 to 329.
In FIG. 10, reference numeral 111 denotes a quasi-coherent detector; reference numeral 112 denotes a correlation value calculator; reference numerals 113 and 114 denote square calculators; reference numeral 115 denotes an adder; reference numeral 411 denotes a code synchronization point detector; reference numeral 133 denotes a symbol clock generator.
The quasi-coherent detector 111 in this conventional initial acquisition circuit generates an in-phase component (a real component) and an orthogonal component (an imaginary component) of a complex spread spectrum signal from a received RF signal.
The in-phase component and the orthogonal component of the complex spread spectrum signal generated by the quasi-coherent detector 111 are correlatively operated by the correlation value calculator 112 with the identical spread code sequences that are multiplied by the received RF signal, so that an in-phase correlation value and an orthogonal correlation value are calculated. The in-phase correlation value and the orthogonal correlation value output from the correlation value calculator 112 unit are squared by the square calculators 113 and 114. Thereafter, these values are added to each other by the adder 115 to calculate a square correlation value. Further, an acquisition pulse representing a code synchronous position is generated in the code synchronization point detector 411 when the square correlation value has the maximum value within one spread code cycle.
Finally, a symbol clock synchronized with the timing of the acquisition pulse is generated in the symbol clock generator 133. Code synchronization between the received RF signal and a spread code used in a correlator in the reception device can be established using this symbol clock.
The procedure explained above will be explained again using equations. When a transmission carrier wave angular frequency is represented by .omega..sub.c, a digital information signal at time t is represented by D.sub.m (m=1, 2, 3, . . . ), and a spread code sequence is represented by c.sub.i ={c.sub.0, c.sub.1, c.sub.2, . . . , c.sub.L-1 } the received RF signal f(t) can be expressed as: EQU f(t)=D.sub.m c.sub.i cos(.omega..sub.c t)+jD.sub.m c.sub.i sin(.omega..sub.c t).
In the following explanation, it is assumed that the frequency of a local carrier wave generated by the VCO 341 is .omega..sub.c which is equal to the transmission carrier wave angular frequency. Since the received RF signal is multiplied by the local carrier wave generated by the VCO 341 in the multiplexer unit 343, a signal r(t) filtered by the lowpass filter 345 can be expressed as: EQU r(t)=D.sub.m c.sub.i cos(.DELTA..theta.).
Similarly, a signal i(t) multiplied in the multiplexer 344 and filtered by the lowpass filter 346 can be expressed as: EQU i(t)=D.sub.m c.sub.i sin(.DELTA..theta.).
Here .DELTA..theta. represents a carrier phase error between the transmission and the reception sides.
It is considered that r(t) and i(t) are correlatively operated with identical spread code sequences c.sub.i '={c.sub.0 ', c.sub.1 ', c.sub.2 ', . . . , c.sub.L-1 '} as those used in the transmission device in the in-phase correlation value calculator 351 and the orthogonal correlation value calculator 352 respectively.
If the phases of the spread code sequence c.sub.i ' and the spread code sequence c.sub.i are different from each other, a sum of squares of the in-phase component r(t) and the orthogonal component i(t) is small. If the phases of the spread code sequence c.sub.i ' and the spread code sequence c.sub.i are equal to each other (code synchronization is established), the sum of squares of the correlation values is large because a code sequence used as spread codes has the following characteristic feature. That is, an autocorrelation value obtained when the phases of the code sequences are equal to each other is large, and a autocorrelation value obtained when the phases of the code sequences are not equal to each other is small. Therefore, the sum of squares of the correlation values of the in-phase component and the orthogonal component is large when code synchronization is established, and the sum of squares when code synchronization is not established is small. Therefore, if the sum of squares of the correlation values is calculated, a code synchronization state between a complex spread spectrum signal and a spread code used in the correlator of the reception device can be established from this sum.
When the conventional initial acquisition unit 141 explained above is used in the identical spread code multiplex SS system, parallel data sequences multiplied by identical spread code sequences and delayed are multiplied to perform data transmission. Therefore, in the correlator on the reception device side, as shown in FIG. 11, square correlation peak values of the number of times of multiplexing are exhibited within one symbol cycle. FIG. 11 is a timing chart of a square correlation value and a symbol clock when the number of times of multiplexing n=3. Thus, when the conventional initial acquisition circuit is used in the identical spread code multiplex SS system, it is disadvantageously difficult to detect a code synchronization point of the entire system (code synchronization point obtained when no delay times are not given by the spread modulator 22(1) to 22(n) in the transmission device described in the prior art) to generate a single clock.
As a method of generating a symbol clock in the identical spread code multiplex SS system, for example, a scheme using a spread code for code synchronization has been explained in Japanese Patent Application Laid-Open No. 4-360434. However, in addition to the correlation calculators 351 and 352 for data communication, another pair of correlation calculators for code synchronization are required (one for calculating an in-phase correlation value and the other for calculating an orthogonal correlation value). Thus, the circuit scale disadvantageously increases.